Combustor for high flow velocities



A ril'zl, 1953 B. SZCZENIOWSKI 2535 424 COMBUSTOR FOR HIGH FLOWVELOCITIES Filed Jill 50, 1948 INVENTOR Patented Apr. 21, 1953 UNITEDSTATES PTENT OFFICE Application July 30, 1948, Serial No. 41,565 InCanada August 2, 1947 3 Claims.

My invention relates to a combustor which creates a homogeneous air-fuelmixture and enables this mixture to be burnt while flowing withconsiderable velocity through a relatively short duct, i. e., theinvention enables one to use a limited combustion space. While acombustor of this type may of course be used in various ways, it hasbeen found especially suitable for jet engines in which application thespace available for the combustor is strictly limited and a weightycombustor is undesirable.

The velocity of combustion, i. e. the velocity of displacement of theflame front cannot be precisely defined before the physical and chemicalconditions of combustion are specified. In a closed vessel in which themixture is ignited at a certain point, the flame is propagated in alldirections, starting from this point, and at a certain moment the Wholevolume of the vessel is divided into two parts by the flame front: oneburnt and the other not yet burnt. The velocity of flame propagation isvariable here and depends not only on the initial conditions, i. e. onthe pressure and temperature of the unburnt mixture, its chemicalcomposition and its degree of homogeneity, but also on the fact that thepart already burnt increases the velocity of the flame propagation inexpanding, this expansion being due to the rise in temperature; thus thepart not yet burnt is at the same time being compressed. Thus, anadditional displacement of the flame front takes place, while thepressure in both parts, burnt and unburnt, increases, this latter factchanging the physico-chemical conditions influencing the velocity offlame propagation. The form of vessel also influences the process, asthe rate of heat release is proportional to the volume burnt per unit oftime, the combustion being assumed to propagate equally in alldirections, thus resulting in a flame front of spherical form, whichcondition will only actually occur if the gas is at rest and isperfectly homogeneous.

In order to eliminate all influences except the initial physico-chemicalconditions, an abstractive definition of the velocity of flamepropagation is formulated as follows: a long tube of constantcross-section, insulated thermally from surroundings, open at one endand closed at the other, is filled with perfectly homogeneous airfuelmixture. Ihis mixture is ignited at the open end of the tube,simultaneously all over the crosssection. The surplus volume of theburnt gases may escape freely at the rear, through the open end; thus,the influence of compressing the unburnt part of the mixture by theburnt gases is non-existent. The velocity of displacement of flamefront, so defined, is the specific velocity of flame propagation.

This specific velocity of flame propagation depends on three mainfeatures:

(1) Chemical composition of a mixture, 1. e. kind of fuel and air excesscoeificient;

(2) Degree of homogeneity of the air-fuel mixture, i. e. degree ofmixing;

(3) Initial physical conditions, i. e. pressure and temperature of theunburnt mixture.

The specific velocity of flame propagation is actually very low, e. g.in a chemically correct air-gasoline mixture and in standard atmosphericpressure-temperature conditions, it is of the order of 4 feet persecond. If in the same conditions the kind of fuel changes, the specificvelocity of flame propagation changes also within certain limits, beingfor instance a little higher for ethyl alcohol, considerably higher forhydrogen, etc.

The specific velocity of combustion depends solely on the probability ofthe meeting between molecules of vaporised fuel with an adequate numberof oxygen molecules. If, therefore, the mixture is homogenous andsupposedly at rest, the phenomenon is conditioned only by the meandistance between the molecules which have to meet on the one hand, andby the mean molecular velocity on the other. In other words, the onlyphysical parameters influencing the phenomenon are pressure andtemperature of the unburnt mixture, according to the kinetic theory ofgases.

The air excess coefiicient influences considerably the specific velocityof combustion. The maximum of this velocity occurs in a mixture a littlericher than chemically correct, while it drops very quickly when the airexcess coefficient increases or decreases, e. g. for 50 per cent airexcess, this velocity is only 40 per cent of that for a chemicallycorrect mixture. This must be so, as in both cases the mean distancebetween the molecules having to meet increases.

This mean distance increases. also when the mixture is not homogeneous,all other conditions remaining the same. The degree of homogeneity is,therefore, very important for attaining the possible maximum of velocityof flame propagation, and any means to attain the high degree ofhomogeneity in an actual combustor are desirable, provided this does notcause too great losses of energy (e. g. loss in pressure in a duct). Oneofsuch means is a vortex created artificially in a flow, either beforeor during combustion. But even an ideal homogeneity of the mixture willonly permit the attaining of the value of specific velocity ofcombustion as theoretically expected in an ideal case, this value beingvery low. No vortex can permit us to exceed this velocity in actualpractice, as such a vortex may be only of the order of a few hundredfeet per sec. while the mean molecular velocity is of the order of afew, and maybe several thousand feet per sec.; thus, the resultantvelocity vector remains practically unaffected, the same holding truefor the probability of meeting of molecules.

The only adequate solutions seem to be: (1) considerable lowering of thevelocity of flow of mixture; (2) considerable shortening of the distanceto be run over by a flame.

The first solution is in common use. However, it increases considerablythe bulk and weight of any combustor-all the more, because a properlydesigned diffuser must be applied to lower the flow velocity with aminimum of loss in dynamic head.

In order to explain the other solution, consider a tube of constantcross-section in which airfuel mixture is flowing with a velocity of,say, 160 ft./sec. The mixture is ignited at a certain point lying on thetube axis. According to the theoretical principles, in a mixture at restthe flame is propagated at a constant velocity in all directions alongstraight lines starting from the point of ignition, the surface of theflame being spherical. This propagation remains the same if the wholemass of mixture is in motion with a certain constant velocity, in ourcase 160 ft./sec.; but here the flame velocity must be regarded asrelative to the mass of mixture supposed to be at rest. The flame is,therefore, drawn by the flow velocity.

The combustion is not ended until the flame reaches the tube wall, i. c.it must cover a distance equalling the tube radius. Let the tubediameter be 6 inches; thus, the necessary lapse of time will be sec.,provided the specific velocity of combustion is 2 ft./sec. During thesame lapse of time the flame would be drawn by the flow velocity alongthe tube to a distance of 10 feet if the flow velocity were constant; infact this velocity must increase with the temperature and the figure of10 feet per see. is practically greater. Thus, in order to accomplishcombustion inside the tube, its length must be at least twenty times itsdiameter.

The principle of my invention is as follows:

In order to shorten a distance to be traversed by a flame in thecombustor, flames are caused to issue at a plurality of points which aredistributed equi-distantly over the cross-section of the tube. This isdone by a burning gas brought to the flame distribution points from aspecial small burner, in which only a low percentage of the whole heatoutput is released. This auxiliary burner may be of any kind, forinstance of a kind in which the flow velocity is only considerablylowered; actually, this does not affect the weight and bulk of the wholecombustor, as it is a question of a small auxiliary device.

A further feature of my invention is a device whose aim is to secure avery homogeneous airfuel mixture. For this reason the fuel is previouslyvaporised, before mixing with air, in a vaporiser, this latter receivinga small part of the heat contained in the whole mass of gases, during orjust after combustion. The vaporized gas is led to a distributor whichis similar to the device which distrlbutes the ignition flame, but is '4located in the duct upstream of the ignition flame distributing device.The vaporised fuel enters in contact with the air in many points,distributed equidistantly over the tube cross-section, thus enabling theair-fuel mixture to become very homogeneous.

Vaporisation of fuel not only facilitates in rendering the mixturehomogeneous, but also precludes all possibility of icing during fuelspray.

The construction of my combustor is as shown in the accompanyingdrawing, in which:

Figures 1 and 2 represent respectively vertical and horizontallongitudinal sectional views, as obtained in cutting by two planesperpendicular to each other and passing through the axis of symmetry.

Fig. 3 is a perspective view, partly broken, of one of tubes 6 and I8 asappearing in Figures 1 and 2.

Fig. 4 is half of a sectional view showing another kind of auxiliaryburner, different from that shown in Fig. 1.

Fuel comes from fuel tank I, through tube 2, under desirable pressure,to the vaporiser 3. It next passes, in the form of vapor, through tube 4to the fuel distributor and mixer 5. Here the fuel gas passes from theannular chamber to the set of parallel small tubes 6, whosecross-section is in the form of airfoil, in order to reduce the drag.These tubes are distributed equidistantly over the cross-section of themain air duct 1, air entering at 8 in the direction of the arrow. One ofthe tubes 6 is shown in larger scale in Fig. 3. Along the trailing edgeof every such tube a set of equidistantly distributing incisions 9 ismade, the distance between two adjacent incisions equalling that betweentwo adjacent tubes; fuel vapor escapes through these incisions andenters the main air stream.

In order to facilitate the thermal dilatation of tubes 6, they are madeof two parts, l0 and H, one sliding in the other.

A small part of the air output enters the bypass i2; it is compressed indiffuser [3 (this additional compression serving to overcome theaerodynamic resistance in ignition flame distributor [1), while itsvelocity diminishes considerably. A small part of the fuel vapor passesfrom distributor 5, through tube I4, to combustion chamber 15 of theauxiliary burner and mixes with air. The mixture is then ignited byspark plug I6. The velocity of stream in I5 is chosen so low as tosecure continuous combustion without any artificial means to maintainit, spark plug l6 serving only for starting. Burning gases from theauxiliary burner then pass to the annular ignition flame distributor l1,and next to tubes [8, set similarly to tubes 6 and being of similarshape and construction. Ignition flame distributor I1 is placed at adistance from 5, at which homogeneity of the air-fuel mixture is alreadyattained. Burnt gases leave the combustor'at IS.

The kind of source of ignition flame being in principle immaterial,solution of auxiliary burner as described serves only as an example.Another possible solution, simpler in principle, is shown in Fig. 4.Here no auxiliary burner exists, and ignition flame is borrowed from themain flow of burning gases. A small part of the flaming gas escapesthrough tube 2|, whose inlet end 22 is set against the main flowdirection. Then these gases pass through diffuser 23, in order to createsuch increase in pressure as may be necessary to offset the resistanceto flow in ignition flame distributor l7 and tube Hi. In such asolution, the starting spark plug may be placed either at 2 3 or at 25.

Tubes [8 are made from tungsten sheets, in order to protect them againsthigh flame temperature. This temperature has to be high in order tocertainly exceed the flash point of the mixture. Its value may becontrolled by controlling the quantity of fuel vapor entering auxiliarycombustion chamber [5, i. e. the air excess coefficient in thisauxiliary device. Even a chemically correct mixture may be applied hereif necessary, but in this case it is preferable to manufacture all partsof the auxiliary burner from tungsten or, at least, from high qualitystainless steel.

Tube 20 serves to feed the auxiliary burner with fuel at the moment ofstarting,

The distance between tubes IB is to be chosen according to practicalconsiderations. The shorter this distance, the shorter becomes thelength of the main duct 4; but if the number of tubes I8 is too great,the aerodynamic resistance they offer may increase beyond the admissiblelimit. From this viewpoint, the ratio of profile Width I8 and of thedistance between two adjacent profiles must be kept as small aspossible. It is, therefore, advisable to apply profiles which aresurficiently slender and as small as feasible.

What I claim as my invention is:

1. A combustor for high-flow velocities of the type described,comprising a duct of circular cross-section, said duct being open at oneend to receive air and at the opposite end to discharge the products ofcombustion; a fuel tank; a vapporizer mounted in heat transferringrelationship with said duct; two annular chambers concentrically mountedupon said duct; a plurality of open-ended tubes of airfoil cross-sectionparallelly disposed and mounted in the wall of the duct in the region ofeach annular chamber, the open ends of the tubes entering the annularchamber associated therewith; a plurality of perforationsin the trailingedge of each of the said tubes of airfoil cross-section; means fordelivering fuel from said tank to said vaporizer; means for deliveringvaporized fuel from the vaporizer to one of said annular chambers, andthence to the tubes associated therewith, the perforations in said tubesconstituting means whereby fuel may escape from the tubes and mix withair entering the open inlet end of the duct; ignition means; and meansfor leading a relatively small quantity of vaporized fuel mixed with airpast said ignition means; means for delivering the burning mixtureproduced by ignition of the fuel led past the ignition means, to theother annular chamber, and the tubes associated therewith, whereby thesaid burning mixture enters the duct through the perforations in saidtubes at a plurality of points; the perforations in each member of eachseries of tubes being equidistant from one another, and the tubes beingspaced apart from one another a distance equal to the distance betweenan adjacent pair of perforations in one of the tubes, so that the saidburning mixture enters the duct at a plurality of points equidistantlyspaced over the cross-section of the duct.

2. A combustor as defined in claim 1, in which the tubes of airfoilcross-section are each made up of two parts, one part beingtelescopically receivable within the other to accommodate thermaldilatation of the combustor.

3. A combustor as defined in claim 1, in which the means fordeliveringthe said burning mixture to the said other annular chamberincludes a diffuser having a cross-section which increases withproximity to the said other annular chamber.

B. SZCZENIOWSKI.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,404,335 Whittle July 16, 1946 2,422,213 Smith June 17, 19472,458,497 Bailey Jan. 11, 1949 FOREIGN PATENTS Number Country Date357,797 Germany Sept. 1, 1922 554,906 Germany Nov. 2, 1932

